如何在没有外部库的情况下使用 JavaScript 创建简单的 SOAP 客户端？

function soap() {
    var xmlhttp = new XMLHttpRequest();
    xmlhttp.open('POST', 'https://somesoapurl.com/', true);
    var sr = '<?xml version="1.0" encoding="utf-8"?>' +
        '<soapenv:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:api="http://127.0.0.1/Integrics/Enswitch/API" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/">' +
        '<soapenv:Body>' +
        '<api:some_api_call soapenv:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/">' +
        '<username xsi:type="xsd:string">login_username</username>' +
        '<password xsi:type="xsd:string">password</password>' +
        '</api:some_api_call>' +
        '</soapenv:Body>' +
        '</soapenv:Envelope>';
    xmlhttp.onreadystatechange = function () {
        if (xmlhttp.readyState == 4) {
            if (xmlhttp.status == 200) {
                alert(xmlhttp.responseText);
            }
        }
    }
    xmlhttp.setRequestHeader('Content-Type', 'text/xml');
    xmlhttp.send(sr);
}